Non-volatile memory (NVM) is a memory device that retains content stored therein even when power is removed. EEPROM and flash memory are two commonly used non-volatile memory devices. In particular, flash memory has become widely used in electronic devices, especially portable electronic devices, because of its ability to provide data storage at low power levels. Modern day flash memory devices are typically implemented using a floating gate MOS transistor device as the memory cells. A floating gate MOS transistor device includes a floating gate that is formed between a control gate and the channel region (the substrate) of the MOS device and at least partially vertically aligned with the control gate. Charge storage on the floating gate determines the stored data state (“0” or “1”) of the memory cell.
A flash memory cell is read by applying a gate bias to the control gate and sensing the stored data state on the drain terminal of the flash memory cell, which is coupled to the bit line of the memory array. To enable fast read access, the sensing of the stored data on the bit line is typically accomplished using a sense amplifier which compares the current on the bit line with a reference current. Conventional flash memory device uses a reference flash memory cell having the same construction as the flash memory cell in the memory array to generate the reference current. However, the reference flash memory cell and associated circuitry consumes large silicon real estate. Furthermore, a flash memory cell can suffer from cell disturb issue after the flash memory cell has been subjected to many excitation cycles. In particular, since a reference flash memory cell is used for the read operation of multiple memory cells and the reference flash memory cell is thus put through the read operation for a large number of cycles as compared to the memory cells themselves. After many read cycles, the reference memory cell may become degraded so that the reference flash memory cell does not generate the same reference current anymore. The changing or drifting of the reference current over the operational life of the flash memory device is undesirable as it affects the read access time of the memory cells. In some cases, a flash memory device uses an NMOS transistor to generate the reference current. However, the reference current thus generated has a large variation as a function of temperature.